In general, motorized vehicles, such as the conventional automobile, include a powertrain that is comprised of an engine in power flow communication with a final drive system (e.g., rear differential and wheels) via a multi-speed power transmission. Hybrid type powertrains generally employ an internal combustion engine (ICE) and one or more motor/generator units that operate either individually or in concert to propel the vehicle. That is, power output from the engine and motor/generators are transferred through planetary gearing in the multi-speed transmission for communication to the vehicle's final drive system. The primary function of the transmission is to regulate speed and torque to meet operator demands for vehicle speed and acceleration.
Most automatic transmissions include a number of gear elements, generally in the nature of one or more epicyclic planetary gear sets, for coupling the transmission's input and output shafts. Traditionally, a related number of hydraulically actuated torque establishing devices, such as clutches and brakes (the term “torque transmitting device” often used to refer to both clutches and brakes), are selectively engageable to activate the aforementioned gear elements for establishing desired forward and reverse speed ratios between the transmission's input and output shafts. Engine torque and speed are converted by the transmission, for example, in response to the tractive-power demands of the motor vehicle.
Shifting from one speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing one or more “off-going” clutches associated with the current or attained speed ratio, and applying one or more “on-coming” clutches associated with the desired or commanded speed ratio. To perform a “downshift”, a shift is made from a low speed ratio to a high speed ratio. That is, the downshift is accomplished by disengaging a clutch associated with the lower speed ratio, and engaging a clutch associated with the higher speed ratio, to thereby reconfigure the gear set(s) to operate at the higher speed ratio. Shifts performed in the above manner are termed clutch-to-clutch shifts, and require precise timing in order to achieve high quality shifting.
To operate properly, most power transmissions require a supply of pressurized fluid, such as conventional transmission oil. The pressurized fluid may be used for such functions as cooling and lubrication. The lubricating and cooling capabilities of transmission oil systems greatly impact the reliability and durability of the transmission. Additionally, multi-speed power transmissions require pressurized fluid for controlled engagement and disengagement, on a desired schedule, of the various torque transmitting mechanisms that operate to establish the speed ratios within the internal gear arrangement.
Transmissions are traditionally supplied with hydraulic fluid by a wet sump (i.e., internal reservoir) oil system, which is separate from the engine's oil system. The fluid is typically stored in a main reservoir or main sump volume where it is introduced to a pickup or inlet tube for communication to one or more hydraulic pumps. In hybrid-type transmissions, it is conventional practice to have one hydraulic pump assembly that is driven by the engine (e.g., via the engine crankshaft) for supplying hydraulic pressure to the transmission control system. It is also conventional practice to have an additional pump which is driven from alternate power sources so that pressure is available when the engine is not running and the vehicle is in motion.
The various hydraulic subsystems of a power transmission are typically controlled through operation of a hydraulic circuit, also known as a hydraulic control module. The hydraulic control module, in collaboration with an electronic control unit, regulates the flow of pressurized fluid for cooling and lubricating the transmission components, and the selective pressurization of the various torque-transmitting mechanisms to enable transmission shifting and vehicle braking. The hydraulic control module traditionally engages (actuates) or disengages (deactivates) the various transmission subsystems through the manipulation of hydraulic pressure generated within the transmission oil pump assembly with a plurality of valves. The valves used in a conventional hydraulic control circuit commonly comprise electro-hydraulic devices (e.g., solenoids), spring-biased accumulators, spring-biased spool valves, and ball check valves.